Vacuum-protective spark gap with trigger electrode



June 27, 1967 A. A. ROBINSON 3,328,632

VACUUM-PROTECTIVE SPARK GAP WITH TRIGGER ELECTRODE Filed Aug. 16, 1965 s Sheets-Sheet 1 [L30 28 I :1 13 E- 22 l 53 Ih-QO 27 9 7;. L E812 2 3 ---23 r 14 I l fi'hgze 16 FIG.2

June 27, 1967 A. A. ROBINSON 3,328,632

VACUUM-PROTECTIVE SPARK GAP WITH TRIGGER ELECTRODE s SheetS Sheet 2 Filed Aug. 16, 1965 FIG.3

J 27, 1967 A. A. ROBINSON VACUUM-PROTECTIVE SPARK GAP WITH TRIGGER ELECTRODE Filed Aug. 16, 1965 3 Sheets-Sheet 5 FIG.?

A.C. LINE IMG FIG. 6

FIG.5

United States Patent 3,328,632 VACUUM-PROTECTIVE SPARK GAP WITH TRIGGER ELECTRQDE Alfred Alexander Robinson, Stafford, England, assignor to The English Electric Company Limited, London,

England, a British company Filed Aug. 16, 1965, Ser. No. 480,000 11 Claims. (Cl. 315-60) This invention relates to protective spark gap devices.

According to this invention, a protective spark gap device for an AC. transmission system includes a pair of stationary main electrodes insulated from one another in a chamber under high vacuum, one main electrode being adapted to be connected to a high-voltage A.C. power line and the other main electrode being adapted to be connected to earth, the gap between the main electrodes being such that, when the main electrodes are so connected, it does not break down under the normal line voltage, and a pair of satisfactory trigger electrodes having a capacitance between them, each trigger electrode being spaced from the corresponding main electrode by a trigger gap, the trigger electrodes being adapted to be connected each through a resistance to the high-voltage power line and to earth respectively, and the trigger gaps being such as to give field emission in vacuum at a voltage substantially less than the maximum permissible line overvoltage, the arrangement being such that if an impulse voltage appears across the gap between the main electrodes, in addition to the normal line voltage, a current will flow through the capacitance and a substantial proportion of the impulse voltage will appear across the resistances, and thus across the trigger gaps, which will initiate breakdown of the gap between the main electrodes.

A number of embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, of which:

FIG. 1 shows diagrammatically one embodiment of protective spark gap device in accordance with the invention,

FIG. 2 shows a second embodiment, which is a modification of that of FIG. 1,

FIG. 3 shows a third embodiment in which capacitor and resistor elements are external,

FIG. 4 shows the equivalent electrical circuit diagram of the embodiments of FIGS. 1-3,

FIGS. 5 and -6 are diagrams showing voltage and current waveforms in operation of such a device, and FIG. 7 is a schematic view of the invention showing a plurality of spark gap devices in series.

Referring now to FIG. 1, a protective spark gap device for use on high-voltage A.C. transmission systems has a pair of main electrodes 11, 12, each of which is connected by a cylindrical conductor 13, 14 to a correspond ing baseplate 15, 16. The baseplates conveniently provide the terminals for the protective spark gap unit, and are thus connected respectively to the high-voltage power line and to earth. The main electrodes 11, 12 are annular and coaxial with one another, the gap 17 between them being set so that it does not break down under normal line voltage conditions.

Trigger electrodes 18, 19 are provided coaxially with-in the main electrodes 11, 12; the gaps 20, 21 between the trigger electrodes and the corresponding main electrodes are set during manufacture according to a criterion to be described below. The trigger electrodes 18, 19 are electrically connected through resistances 22, 23, .to the corresponding main electrodes 11, 12, and may be supported from the baseplates 15, 16 by insulators 24, 25. The resistances 22, 23 may either be separate elements, or conducting coatings on the insulators, or other known or convenient devices which produce the same effect.

3 ,328,632 Patented June 27, 1967 The facing areas of the trigger electrodes 18, 19 together form a capacitor 26. Typical values of the resistances 22, 23 would be 10 to kilohms and of the capacitance 10-1 0 0 picofarads.

The values of capacity and resistance required are such that a trigger gap current (followed its breakdown) of the order of several amperes is desirable for reliable operations.

For the majority of system voltages and impulse rates of rise this consideration leads to a C XR value in the range 250 kilohm picofa-rads to 25,000 kilohm picofarads.

The electrode arrangement is enclosed in a vacuum envelope which may include a pair of dished, shedded insulators 27 bonded through adaptors 28 to the baseplates 15, 16, and bonded togther at their wider ends. A shield 29 may be provided, bonded to the insulators 27, to protect them from the arc.

Means is provided, as indicated at 30, for evacuating and sealing off the vacuum envelope at a pressure of the order of 10 mm. Hg, or lower.

Referring now to 'FIG. 2, in which the same reference numerals indicate the same parts, the protective spark gap is the same as that shown in FIG. 1 except that the trigger electrodes 38, 39, instead of being flat, are formed with interleaved portions to increase the value of the capacitance between them.

As shown, the upper trigger electrode 38 has a central, circular projection 40 and a coaxial cylindrical projection 41; the latter has an annular flange 42 to define the gap 20 between the trigger electrode 38 and the main electrode 11. The lower trigger electrode 39 has a cylindrical pro jection 43 which is interleaved between the projections 40, 41, with the appropriate spacing.

In the embodiment of FIG. 3, the resistance and the capacitor form part of an external network. Again, the same reference numerals denote the same parts. In this embodiment the main electrodes 51, 52 are cylindrical, are supported from the baseplates 15, 16 by conductors 53, 54, and are coaxially within annular trigger electrodes 58, 59. The bases of the latter are formed with annular flanges 60, 61, which are secured between parts of the insulators 27 so that they may be connected through the external resistances 62, 63 to the corresponding baseplates 15, 16, and thus, through the conductors 53, 54 to the main electrodes.

An external capacitor 66 is connected between the trigger electrodes 58, 59 as part of the external network. The resistances 62, 63 have the same values as resistances 22, 23, and the capacitor 66 has the same value as capacitor 26.

The main electrodes are made of gas-free arc-resistantmaterials.

In the equivalent electrical circuit diagram of FIG. 4, the same reference numerals are used as in FIG. 1, but it will be understood that this figure is also equivalent to the construction shown in FIGS. 2 and 3. If the main spark gap 17 is set so that it does not break down under normal line voltage conditions (for example, 132 kv. or 400 kv. at a frequency of 50 c./s.) and the capacitor 26 (or 66) has the value stated, the current flow through the capacitor will be negligible (less than 1 ma), and the voltages across the resistances 22, 23 (or 62, 63) will be negligibly small. The trigger gaps 20, 21 are set during manufacture to give field emission in vacuum at a voltage substantially less than the line voltage, so that if an impulse voltage having a high rate of rise appears across the main spark gap 17 in addition to the normal-frequency line voltage, a current will flow in the capacitor 26 (or 66) proportional to the differential of the voltage across the main gap with respect to time, and a large proportion of the impulse voltage will therefore appear across the resistances 22, 23 (or 62, 63); the voltage across resistance 22 (or 62) also appears across the trigger gap 20- and that across resistance 23 (or 63) across the trigger gap 21. Field emission and subsequent cathode spot formation at the trigger gaps 20, 21 will initiate r-apid breakdown of the main spark gap 17, thus protecting the line from the impulse voltage.

It will be understood that the rate of rise of an impulse voltage resulting, for example, from a lightning strike on the line is very much greater than the rate of rise of voltage in normal operation; the difierence may be of the order of 100:1.

Following breakdown of the main spark gap 17, a normal-frequency loop of current will flow, and extinction of the arc and voltage recovery will occur at the next available current zero.

In FIGS. 5 and 6, V represents the line voltage, I the current flowing in capacitor 26 (or 66) and I the current flowing across the main gap 17. FIG. 5 shows the waveforms under normal line voltage conditions, 1 being very small and I zero.

In FIG. 6 the waveforms are shown when an impulse voltage is applied to the line; the solid trace shows the voltage as limited by the protective spark gap device, and the dotted trace the voltage which would otherwise occur; the recovery voltage is also shown. The current 1 flowing in the capacitor may rise to a value of the order of 1.0 A; the third diagram of FIG. 6 shows the arc extinction at the next current zero.

A number of protective spark gap units 67 (FIG. 7) may, if desired, be employed in series, and by this means and by suitable provision for gap adjustment during manufacture, lines having a wide range of system voltages may be protected by the use of standard units.

The device may also be employed to protect from surges individual items of apparatus, e.g. transformers and reactors.

What I claim as my invention and desire to secure by Letters Patent is:

1. A protective spark gap device for an AC. trans mission system, including a pair of stationary main electrodes insulated from one another in a chamber under high vacuum, one main electrode being adapted to be connected to a high-voltage AC. power line and the other main electrode being adapted to be connected to earth, the gap between the main electrodes being such that, when the main electrodes are so connected, it does not break down under the normal line voltage, anda pair of stationary trigger electrodes having a capacitance between them, each trigger electrode being spaced from the corresponding main electrode by a. trigger gap, the trigger electrodes being adapted to be connected each through a resistance to the high-voltage power line and to earth respectively, and the trigger gaps being such as to give field emission in vacuum at a voltage substantially less than the maximum permissible line overvolt-age, the arrangement being such that if .an impulse voltage appears across the gap between the main electrodes, in addition to the normal line voltage, a current will flow through the capacitance and a substantial proportion of the impulse voltage will appear across the resistances, and thus across the trigger gaps, which will initiate breakdown of the gap between the main electrodes.

2. A protective spark gap device as claimed in claim 1 in which the main electrodes are annular and co-axially surround the respective trigger electrodes to form annular trigger gaps.

3. A protective spark gap device as claimed in claim 1 in which the trigger electrodes are annular and coaxially surround the respective main electrodes to form annular trigger gaps.

4. A protective spark gap device as claimed in claim 1 wherein said capacitance is formed directly between the trigger electrodes.

5. A protective spark gap device as claimed in claim 4 wherein one trigger electrode is formed with an annular projection which projects into an annular groove in the other trigger electrode.

6. A protective spark gap device as claimed in claim 1 wherein said capacitance is outside said chamber.

7. A protective spark gap device as claimed in claim 6, wherein said stationary trigger electrodes are annular, coaxially surround the respective main electrodes to form annular trigger gaps, and are each formed with an annular flange which passes through the wallv of said chamber, and said capacitance is connected between said annular flanges.

8. A protective spark gap device as claimed in claim 1 wherein the resistances are inside the chamber.

9. A protective spark gap device as claimed in claim 8, wherein the resistances are in the form of conductive coatings on insulators by which the trigger electrodes are supported.

10; A protective spark gap device as claimed in claim 1, wherein the resistances are outside said chamber.

11. A plurality of protective spark gap devices as claimed in claim 1, said devices being connected electrically in series between a high-voltage AC. power line and earth.

No references cited.

JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

1. A PROTECTIVE SPARK GAP DEVICE FOR A.C. TRANSMISSION SYSTEM, INCLUDING A PAIR OF STATIONARY MAIN ELECTRODES INSULATED FROM ONE ANOTHER IN A CHAMBER UNDER HIGH VACUUM, ONE MAIN ELECTRODE BEING ADAPTED TO BE CONNECTED TO A HIGH-VOLTAGE A.C. POWER LINE AND THE OTHER MAIN ELECTRODE BEING ADAPTED TO BE CONNECTED TO EARTH, THE GAP BETWEEN THE MAIN ELECTRODES BEING SUCH THAT, WHEN THE MAIN ELECTRODES ARE SO CONNECTED, IT DOES NOT BREAK DOWN UNDER THE NORMAL LINE VOLTAGE, AND A PAIR OF STATIONARY TRIGGER ELECTRODES HAVING A CAPACITANCE BETWEEN THEM, EACH TRIGGER ELECTRODE BEING SPACED FROM THE CORRESPONDING MAIN ELECTRODE BY A TRIGGER GAP, THE TRIGGER ELECTRODES BEING ADAPTED TO BE CONNECTED EACH THROUGH A RESISTANCE TO THE HIGH-VOLTAGE POWER LINE AND TO EARTH RESPECTIVELY, AND THE TRIGGER GAPS BEING SUCH AS TO GIVE FIELD EMISSION IN VACUUM AT A VOLTAGE SUBSTANTIALLY LESS THAN THE MAXIMUM PERMISSIBLE LINE OVERVOLTAGE, THE ARRANGEMENT BEING SUCH THAT IF AN IMPULSE VOLTAGE APPEARS ACROSS THE GAP BETWEEN THE AIN ELECTRODES, IN ADDITION TO THE NORMAL LINE VOLTAGE, A CURRENT WILL FLOW THROUGH THE CAPACITANCE AND A SUBSTANTIAL PROPORTION OF THE IMPULSE VOLTAGE WILL APPEAR ACROSS THE RESISTANCES, AND THUS ACROSS THE TRIGGER GAPS, WHICH INITIATE BREAKDOWN OF THE GAP BETWEEN THE MAIN ELECTRODES. 